170 



STEMS 



which supply the plant during the dry season. In the stems of 

 Sorghum and Sugar Cane, so much sugar is accumulated and re- 

 tained that these plants are grown for the sugar which they afford. 

 In the stems of trees much food is stored in the form of starch, 



and when transferred to grow- 

 ing regions during early spring, 

 it is changed to sugar, in which 

 form it occurs in solution in 

 the sap of the tree. The so- 

 called maple sap obtained from 

 the Sugar Maple is a good illus- 

 tration of sap which contains 

 much stored food in the form 

 of sugar. In early spring be- 

 fore the leaves appear, the trees 

 are so gorged with sap that it 

 can be drawn off by boring into 

 the wood and inserting spiles. 

 This sugar comes from reserve 

 food accumulated when the 

 leaves are active, and serves as 

 FIG. 147. A branch of Myrsiphyl- a supply for the growt h of new 

 lum, showing the cladophylls (a), and f u ^ beginning of the 



the scale-like leaves (6). . 



growing season. 



Some stems, notably those of the Irish Potato, contain large 

 amounts of starch on account of which they are valuable for food. 

 Another tuber-like stem similar to that of the Irish Potato is pro- 

 duced by the Jerusalem Artichoke a plant of the Sunflower 

 type and often grown on account of the food value of its under- 

 ground tubers. 



Many of the early spring plants, such as Spring Beauty, Dutch- 

 man's Breeches, Wind Flower, some Violets, and many other 

 plants having a supply of food at hand can spring up quickly, 

 flower, and accumulate another supply of food before the sunlight 

 is excluded by the forest foliage. Such plants, being seen only 

 in April or early May, have what is called the vernal habit, i.e , 

 they live their life cycle in the spring of the year. The food 

 reserve of stems has much to do with the vernal habit. 



Classes of Stems. There are many ways in which stems may 

 be classified. Stems are classified as monocotyledonous or dicoty- 



